Stickandrudderman wrote:
A lot of the Mercedes cars I work on are 1960s with mechanical fuel injection. The story goes (and I have no wish to disprove it) that Me109s had Mercedes engines and they used mechanical fuel injection that had barometric compensation and that same system can be found on the cars that I work on……. Plausible that modern cars have a software equivalent.
Electronic FI can use various different methods (e.g. intake mass flow sensors or absolute pressure sensors) to adjust fuel delivery for reduced density intake air. None of them correct the fundamental issue of reduced air mass flow at altitude, they just reduce fuel delivery to match. That reduction in fuel flow has a proportional effect in reducing power output. Somewhere I have the altitude compensation charts that I used on motorcycle EFI to trim fuel injector pulse time based on the bike’s absolute pressure sensor signal. IIRC a little over half the fuel should be delivered at 12,000 ft for any given at any given throttle position and rpm. Originally on those 1990’s motorcycles the chart was under compensated by the factory meaning the mixture went richer, faster than a carburetted bike with increasing altitude. Apparently the manufacturer was nervous about over compensating and at that time had no laboratory to check their work. I checked it by riding the bike into the mountains, many times.
On that relatively simple open loop EFI system there were similar compensation charts for ambient and engine temperature, but with a standard atmosphere the effect of ambient pressure was dominant when at high altitude.
